his delay allows mission engineers to continue their evaluation of all spacecraft systems and operational procedures after the loss of Mars Climate Orbiter. The thruster maneuver was previously scheduled for today.
The spacecraft is healthy and the team is continuing to test and train for the early surface phase of the mission.
Mars Polar Lander is currently 18.8 million kilometers (11.7 million miles) from Mars, approaching the planet at a speed of 4.8 kilometers per second (10,740 miles per hour) relative to the red planet.
One argument that such a body of water once existed was suggested by features in images from the NASA Viking missions taken in the 1970s, which were interpreted by a number of researchers as remnants of ancient coastlines.
The images from Mars Global Surveyor, taken in 1998, have a resolution five to 10 times better than those that Viking provided. With this closer inspection, none of these features appears to have been formed by the action of water in a coastal environment.
"The ocean hypothesis is very important because the existence of large bodies of liquid water in the Martian past would have had a tremendous impact on ancient Martian climate and implications for the search for evidence of past life on the planet," said Dr. Kenneth Edgett, a staff scientist at Malin Space Science Systems, San Diego, CA, the institution that built and manages the Mars Orbiter Camera onboard the spacecraft.
"The newer images do not show any coastal landforms in areas where previous researchers -- working with lower resolution Viking images -- proposed there were shorelines."
About 2 percent of the Mars Orbiter Camera images were targeted to look in places that would test shorelines proposed by others in the scientific literature.
"Even on Earth, looking for ancient shorelines from the air or space is a challenge," said Dr. Michael Malin, principal investigator for the camera at Malin Space Science Systems.
"Despite these difficulties, we believe these Mars Orbiter Camera images of the proposed shorelines are of a high-enough resolution that they would have shown features indicative of a coastal environment had there been an ancient ocean on Mars."
The paper containing these new conclusions was published in the October 1 issue of the Journal of Geophysical Research Letters.
One area that might have been a coastline is located northwest of the great volcano Olympus Mons. Researchers looking at Viking images have suggested that there might be a cliff separating the western margin of the Lycus Sulci uplands from the lower-elevation, smoother Amazonis plains. The proposed cliff looked like the kind that forms on Earth from erosion as waves break against a coastline.
Three high-resolution images were taken of this proposed coastline. The uplands are roughly textured, while the flat plains appear smoother. The image shows that the contact between the two regions is clearly not a wave-cut cliff, nor are there any features that can be unambiguously identified as coastal landforms, according to Malin.
"While the suggestion that Mars at one time had oceans cannot be ruled out, the foundation for the 'ocean hypothesis' developed in the 1980s on the basis of suspected shorelines appears now to have been incorrect," Malin concluded. "However, it should be understood that there is significant other evidence of water on Mars in the past, both from Mars Global Surveyor and from previous missions."
"Today, the Mars Orbiter Camera continues to acquire new high-resolution pictures, each one helping to search for clues to the very important question of the role of water in the evolution of Mars."
More information and images about the Mars Global Surveyor mission is available at: http://mars.jpl.nasa.gov/mgs/ and http://photojournal.jpl.nasa.gov .
Additional details about the paper and the new Mars images are at: http://www.msss.com/mars_images/moc/grl_99_shorelines/ .
"It is like we are flying from New York to California and with today's maneuver we decided to land in Los Angeles instead of San Diego," said Dr. Sam Thurman, flight manager for the Mars Polar Lander at NASA's Jet Propulsion Laboratory. "During the next few months leading up to landing, we'll conduct additional maneuvers that will further adjust the flight path, similar to deciding which specific runway at LAX we want to land on."
The landing site is located at 76 degrees south latitude and 195 degrees west longitude, near the northern edge of the layered terrain in the vicinity of the Martian south pole. The course correction came as the Lander was 22.7 million miles from Mars, traveling at a speed of 8,300 miles per hour relative to the planet.
"Mars is a cold, dry desert, but our camera has shown it is far from being a stagnant place," said Dr. Michael Malin, principal investigator for the Mars Global Surveyor camera at Malin Space Science Systems, San Diego, CA. "Over the past few months, we have captured a unique record of seasonal and meteorological events, which demonstrates that Mars is active and dynamic today."
The spacecraft's camera monitors the planet's weather on a daily basis from orbit, just like weather satellites on Earth.
Today, Mars is a much more dynamic place than the planet the Viking orbiters and landers saw in the late 1970s. The weather has been particularly active during the past two months, as spring arrived in the southern hemisphere and autumn approached in the north.
"Storm clouds have been brewing over the north polar ice cap all through the month of July, and soon, ever-increasing portions of the north polar cap will be plunged into wintertime darkness," Malin said. "As the season changes rapidly, clouds will cover much of the northern plains and it might begin to snow as the polar cap expands."
In other regions of Mars, dust devils are the prevailing weather story. Dust devils result from spinning vortices of air that arise when the ground is heated and general wind flow is light. On Earth they are relatively small features, but on Mars, dust devils are thought by some to be a major transporter of the fine, pinkish dust that gives the sky its unearthly brownish color, as seen by the Mars Pathfinder and Viking landers. Dust devils may also help initiate the seasonal raising of dust over wide areas of Mars.
In mid-May, swirling columns of dust as high as five miles (eight kilometers) were observed in northern Amazonis Planitia. Dust devils in this area, northwest of the large Tharsis volcanoes, appear to be common; they were also seen by the Viking missions of the 1970s.
The average dust devil is slow- moving and may carry several tons of dust within its height of 1.2 miles (two kilometers).
Each lasts for a few hours at most during the hottest part of the Martian day in the late afternoon, Malin said. Although the winds in these vortices are sufficient to raise dust, they have much less power than tornadoes on Earth, which develop under very different meteorological circumstances.
Global Surveyor's camera has also returned tantalizing evidence of recent shifting sands in dune fields first seen in Mariner 9 pictures of Mars from the early 1970s. Scientists are interested in dune fields isolated within large impact craters because their dark color suggests that the dust which covers much of the rest of the planet does not accumulate on their sandy surfaces.
"This indicates that the dunes must be moving and that over time we may be able to see changes that will allow us to measure the rates of wind erosion on Mars," Malin explained.
Sand dunes also are giving Mars scientists some new insights as to how Mars' seasonal polar ice caps retreat at the end of each winter as seasonal warming occurs. The most dramatic views show patches of dark sand poking through fields of carbon dioxide frost. First seen in 1998 in the north polar region, the same features have been seen this year on dunes near the south pole.
"These pictures look like aerial photographs of dunes on Earth," Malin said. "They are so unusual in this context that we thought for a while that we were looking at a process that involves small 'explosions,' but the new images showed that wind was responsible for the streaks we were seeing." The dark spots on frost-covered dunes continue to grow and spread as spring approaches until, eventually, the entire dune field is frost- free.
A variety of new images of Mars is available on the Internet.